1. Field of the Invention
The present invention relates to a magnetic memory device of a magnetic random access memory (hereinafter referred to as “MRAM”), and more particularly to a magnetic memory device using a magneto-resistive element and a method of manufacturing the magnetic memory device.
2. Description of the Related Art
The MRAM is a generic term of a nonvolatile solid-state memory which can randomly rewrite, store and read information as an information recording medium, making use of a variation in resistance value of a barrier layer due to a change in direction of magnetization of ferromagnetic layers. In general, a memory cell of the MRAM has a stacked structure of a plurality of ferromagnetic layers and the barrier layer. For example, a pin layer, which is a first ferromagnetic layer, a barrier layer which is an insulation layer, and a free layer, which is a second ferromagnetic layer, are stacked. The first and second ferromagnetic layers are configured to sandwich the barrier layer.
The memory cells are provided at intersections of sense lines and word lines. The sense lines and word lines are formed in cross stripes, and the intersections are arranged in a matrix. Each memory cell is disposed such that it is interposed between the associated sense line and word line.
Information is recorded such that binary information “1” and binary information “0” are made to correspond to two states, that is, a state in which the magnetization directions of the pin layer and free layer, which are structural components of the memory cell, are the same (“parallel”) and a state in which the magnetization directions of the pin layer and free layer are opposite (“antiparallel”). The write operation of the information “1”, for example, is performed such that the magnetization direction of the free layer of each memory cell is reversed by a magnetic field that is generated by letting a current flow in the word line so that the magnetization directions are the same (“parallel”). This memory cell is a nonvolatile memory which consumes no power, in principle, at an information retention time and the stored information is retained even if power to the memory cell is turned off.
The information reading operation is performed by detecting the resistance of the barrier layer having a resistance value due to a so-called magneto-resistive effect. This is a phenomenon in which the electric resistance of the barrier layer of the memory cell varies depending on the relative angle between the magnetization directions of the ferromagnetic pin layer and free layer of the memory cell, on the one hand, and a sense current direction, on the other hand, or depending on the relative angle between the magnetization directions of the pin layer and free layer.
Functional differences between the MRAM and a conventional dielectric-based charge-accumulation type semiconductor memory, such as a DRAM, are explained. First, the MRAM is a nonvolatile memory and is capable of rewriting data 1015 times or more. Second, the MRAM is capable of non-destructive read, and is also capable of decreasing a read cycle time since no refreshing operation is required. Third, compared to the charge-accumulation-type semiconductor memory, the MRAM has a higher resistance to radiation with respect to the storage of information.
It is expected that the integration density of the memory cells per unit area and the write/read time of the MRAM are approximately equal to those of the DRAM. Thus, by virtue of the remarkable feature of the complete nonvolatility, it is expected that MRAMs would be applied to external memory devices for mobile equipment, hybrid LSIs, and main memories of personal computers.
A type of MRAM, which is currently under investigation for practical use, uses a magneto-tunneling-junction element (hereinafter referred to as “MTJ element”) in the memory cell (see, e.g. U.S. Pat. No. 5,946,228 and U.S. Pat. No. 6,072,718).
The MTJ element mainly comprises three layers, i.e. a ferromagnetic layer/an insulating layer (tunneling barrier layer)/a ferromagnetic layer. A current flows through the insulating layer by a tunneling effect. The tunneling resistance value of the insulating layer varies in proportion to a cosine of the relative angle of magnetization directions of both ferromagnetic layers. When the magnetization directions of both ferromagnetic layers are antiparallel, the tunneling resistance takes a maximum value. For example, in the case of NiFe/Co/Al2O3/Co/NiFe tunneling junction, a rate of change in resistance value, which exceeds 25%, is found in a low magnetic field of 50 Oe or less.
In a micro-manufacturing process for forming an MTJ element, a combinational process using photo-lithography and Ar ion etching is generally used.
Additionally, in the field of semiconductors, dry etching processes using chemical reactions, such as chemical dry etching (CDE) and reactive ion etching (RIE) are used.
In addition, detailed descriptions of the conventional MRAM structures appear in U.S. Pat. No. 5,946,228 and U.S. Pat. No. 6,072,718.
As has been described above, in order to form the MTJ element, the stacked structure of the magnetic layers and barrier layer for forming the MTJ element has to be subjected to the micro-manufacturing process using ion etching. An ion etching method used in the micro-manufacturing process of the MTJ element is a physical sputtering method. However, if a micro-manufacturing process using ion etching is performed, sputtered particles removed from the surface being processed are produced as a residual in the course of the etching process and is re-deposited to the side of the resist mask, the processed surface of the MTJ element, or the inside of the processing apparatus.
At present, in the etching of Si, SiO2, etc. using chemical reactions such as chemical dry etching (CDE) or reactive ion etching (RIE), the material being etched is removed in a gas phase as a halide having a high vapor pressure. However, halides of 3d transition metals, such as Fe, Ni, Co and Cu, used in the formation of the MTJ element, have a low vapor pressure, and it is difficult to be processed in the ordinary semiconductor manufacturing process.
On the other hand, there is an idea that a mixture gas of carbon monoxide, ammonia, etc. is used and chemical etching is performed by forming an organic metal compound. In this method, however, the chemical reaction rate is insufficient, and the process inevitably involves physical sputtering using a reaction gas. Thus, this method has not yet been put to practical use.
However, according to the micro-manufacturing process using physical sputtering, the aforementioned residual film of processed material remains on the side surface of the processed part of the MTJ element. It has turned out that in some cases, the residual film has electrical conductivity, and it may short-circuit the insulating barrier film, leading to initial defects of the MRAM cell.